Relationship friction, failures and mistakes, moral dilemmas and temptations

The character being spotted by their spouse or child while out with a lover
Former love interests contacting the character's spouse to make claims of infidelity
A significant other discovering a paper trail of hotel receipts, lavish gifts, or text messages
A celebrity character's affair being exposed by the media 
A character confessing to cheating after giving their long-term partner a sexually transmitted disease, being caught in one too many lies, or being overcome with guilt
A character's spouse catching the character and a mutual friend in the act
A former lover leaving the character a large inheritance, leading to difficult conversations and a confession
The appearance of an illegitimate child who couldn't have been conceived with one's partner

The character softening the details of the affair to make it less damning
Telling so many lies the character has trouble keeping track of them 
Having to convince the person who discovered the infidelity to keep it a secret
Trying to continue the extra-marital relationship while enduring much more scrutiny
Trying to do damage control (saying it didn't mean anything, apologizing, etc.) and failing
Discomfort and awkwardness arising from having to admit wrongdoing
Having to temporarily relocate to a hotel or friend's house to give the partner space
The couple deciding to keep the infidelity quiet while they figure out what to do and having to act as if everything is fine
The infidelity being made public, exposing the character's spouse and children to humiliation
The character having to explain their actions to loved ones and face judgment
Being shunned by certain family members or friends
Awkwardness around friends and family members who have learned about the cheating
The character ending the relationship with the other party but still having to see them socially or work with them